In a HST (Hydro Static Transmission) that is one type of hydraulic continuously variable transmission, a hydraulic pump and a hydraulic motor are connected to each other by a closed circuit. In the HST, a rotational frequency (rotational speed) of the hydraulic motor can be controlled by adjusting an discharge flow rate of the hydraulic pump. The hydraulic motor can be stopped by setting the discharge flow rate of the hydraulic pump to zero. Further, the hydraulic pump can switch an discharge direction of operating oil. A rotational direction of the hydraulic motor can be switched by switching the discharge direction. One example of such hydraulic pump is a variable-displacement, both-tilt (over-center) swash plate pump.
According to this swash plate pump, in order to set the discharge flow rate to zero, a tilting angle of a swash plate needs to be kept at 0°. However, it is structurally difficult to continuously keep the tilting angle of the swash plate at 0°. Therefore, the operating oil is ejected from one of two ports of the swash plate pump at all times. As a result, differential pressure is generated between two ports of the hydraulic motor, and the hydraulic motor is continuously driven. To avoid this, the HST includes a neutral control valve disclosed in, for example, PTL 1.
A neutral valve of PTL 1 includes two pressure chambers connected to two respective ports of the swash plate pump, and pressures of the two pressure chambers act on both respective end portions of a spool against each other. When differential pressure between the two pressure chambers is low, the spool is located at a neutral position to make the two ports communicate with each other. With this, the differential pressure which may be generated between the two ports of the hydraulic motor is absorbed, and thus, the driving of the hydraulic motor can be stopped. On the other hand, when the differential pressure between the two pressure chambers is high, the spool moves to disconnect the two ports from each other. With this, energy efficiency deterioration caused when the hydraulic motor is driven with the two ports communicating with each other.